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ADS8867: Confirmation of use case

George Triantafillou
Prodigy 50 points
Part Number: ADS8867
Other Parts Discussed in Thread: THS4521

Tool/software:

Hi, I hope you're doing well.

I need to measure the current of a PWM signal with the following specifications:

  • Period: 0.02 seconds (50 Hz)
  • Pulse width: 450 microseconds

I’ve been considering the ADS8867, particularly the reference circuit shown in Figure 62.

 

I plan on adding a resistor in series and measuring the voltage drop across said resistor on the line. Would this be okay assuming my gain/other calculations are okay?

 

A few more points I would also like to confirm. 

1) with AVDD what is the preferred way to generate this power rail. Can It be the same as my DVDD rail? 

2) In figure 62 is the CONVST pin still being used as shown in this timing diagram as apart of the coms? Or is it being used to input another signal. 

 

Thank you. 

  • 0
    TI__Expert 8796 points

    Hi George,

    Thanks for your question. Regarding the shunt measurement, what kind of voltage drop and common-mode are you expecting? These values must be within the FDA input range.

    Regarding your other questions:

    1. You may tie AVDD and DVDD together and use an LDO or similar. Please ensure the supply rail voltage chosen does not exceed AVDD or DVDD ratings:
    2. I believe CONVST is being using to begin conversions. Not sure what you mean by coms.

    Best regards,

    Samiha

  • 0 in reply to Samiha Sharif
    Prodigy 50 points

    Hello, thank you for the reply. 

    In regards to the shunt measurement. I need to be able to measure in 1mA steps. The signal varies from 1 to 100mA.

    if I use a 5ohm resistor as Rsense My voltage drop across would range from 5mV to 500mV. 

    1) is this range okay? 

    2) Would it be preferred if I use the THS4521 in a configuration like this but with added gain.

     

    The questions regarding 

    AVDD and DVDD, CONVST 

    Thank you that makes sense. 

  • 0 in reply to George Triantafillou
    TI__Expert 8796 points

    Hi George, 

    Both of the configurations look the same, as far as the FDA gain being in feedback. The range sounds appropriate! Please gain it such that you can use more of the ADC dynamic range.

    Best regards,

    Samiha

  • 0 in reply to Samiha Sharif
    Prodigy 50 points

    Thank you for you help.